The Devil and the Deep Blue Sea by Elizabeth O'Roark
You don’t really know a guy until you’ve vacationed with him…
When Drew Wilson’s ex-boyfriend Joel “Six” Bailey asks her to go on his family trip to Hawaii with him just as her life is falling apart, she decides it's the perfect time to give him another chance.
The hitch? The Bailey family includes Six’s rude older brother, Joshua—a hot-nerd doctor who has hated Drew since the moment they met and once suggested she’d steal the family silver.
Drew is determined to win the Baileys over and give this thing with Six a fair shot…but Joshua is making that difficult. Not simply because he is in her way at every turn, but because—as one tropical adventure leads to the next—she’s beginning to wonder if obnoxious, odious Joshua might be the brother she actually belongs with.